The invention relates in general to an apparatus for plasma blasting comprising a driver for supplying pulsed high current to a probe to create a plasma for fracturing a geological formation by shock waves resulting from the plasma. More particularly, the invention relates to an apparatus wherein the driver has a capacitance for storing a large amount of electric charge at high voltage. An inductor of the driver carries the discharge current pulse from the capacitance and delivers it to an electrically matched removable explodable conductor coupled to the probe. The explodable conductor is positioned within a bore of the geological formation or other solid material.
Both exploding wire and spark gap systems are known for producing an explosion or the venting of a propellant gas. Exploding wire systems are exemplified by U.S. Pat. No. 5,052,272 to Lee for Launching Projectiles With Hydrogen Gas Generated From Aluminum Fuel Powder/Water Reactions. Lee discloses a method of generating hydrogen gas with high energy efficiency by applying pulse power techniques to a trigger wire and an aluminum fuel powder-oxidizer mixture. The preferred oxidizer of the aluminum fuel powder is water. The apparatus includes a capacitor bank connected to an induction coil. A metal conductor wire is connected to the induction coil and a fast switch. When the switch is closed, electrical energy from the capacitor bank flows through the inductor and the switch as well as the wire. The total energy of the electrical discharge is preferably from 0.50 to 15 kilojoules per gram of aluminum fuel. The discharge lasts between 10 and 1000 microseconds.
U.S. Pat. No. 3,583,766 to Padberg, Jr. discloses a deep submergence search vehicle having a drill pipe into a bore formed in a layer of mineral deposits and extending into a sedimentary ocean bed. A drill head is positioned at the lower end of the drill pipe with a plasma discharge section positioned above the drill head. An energizing circuit couples electrical energy from a power source to a thin nickel wire extending through the plasma discharge section. When a switch is closed, a high current is suddenly passed through the thin nickel wire exploding it and creating a large plasma discharge accompanied by sharp pressure waves. Openings in the plasma discharge section allow the pressure waves to emerge and produce a rapidly expanding and collapsing gas bubble with accompanying shock waves simulating those of explosives. The alternate bubble expansion and collapse propagates acoustic waves in the form of sharp pressure pulses.
Soviet Union No. SU 357345 A to Yutkin discloses a rock breaking device having a pair of electrodes and a conductive wire strip for insertion in a hole in rock filled with a wetted dielectric bulk material, such as sand, to produce shock waves when energized. The wire is connected to the electrodes and stretched around a dielectric plate. The dielectric plate is positioned in the rock hole for bursting operation.
Spark gap or non-exploding wire systems are exemplified by U.S. Pat. No. 3,679,007 to O'Hare for Shock Plasma Earth Drill which discloses a spark gap probe for drilling deep holes in the earth for the recovery of water or oil. The probe has a center electrode separated from and surrounded by an outer electrode. A condenser or capacitor bank having a capacitance of 400 microfarads and charged to a potential of 6000 volts supplies electrical energy to the electrodes. Shock waves were generated in water the outer surface of the center electrode and the inner surface of the surrounding electrode separated by a gap of 0.75 inch. The center electrode had a diameter of 0.25 inch. The embodiment shown in FIG. 4 has a capacitor or condenser bank charged to 6000 volts or more by the combination of high voltage rectifier and high voltage transformer. In the embodiment shown in FIG. 5 a capacitor bank may be charged to 6000 volts for working in soft earth and higher voltages of 30,000 volts or more for working in harder soil or rocky areas. In each of the embodiments when a switch is closed an initial surge of voltage reaches the electrodes positioned in water. The resistance of the water is lowered as the water is converted to plasma by the electric current pulse. Rapid release of electrical energy across the resistance of the water plasma produces a large amount of heat to produce an explosive effect that impacts and thrusts aside the earth ahead of the electrode.
U.S. Pat. No. 4,741,405 to Moeny et al. discloses a spark discharge drill for subterranean mining. The drill may deliver pulses of energy ranging from several kilojoules up to 100 kilojoules or more to a rock face at the rate of 1 to 10 pulses per second or more. A drilling fluid such as mud or water assists propagation of spark energy to the rock face.
U.S. Pat. No. 4,897,577 to Kitzinger for Electromechanically Triggered Spark Gaps discloses an anode and a cathode having facing surfaces defining a gap. A trigger electrode is located in the vicinity of the gap. A piezoelectric generator connected between the trigger electrode and the cathode triggers the spark gap switch. The switch may handle currents on the order of 100,000 amperes or higher from a capacitor discharge circuit.
U.S. Pat. No. 5,106,164 to Kitzinger et al. for Plasma Blasting Method discloses a plasma blasting process for fragmenting rock in the practice of hard rock mining. Electrical energy from a capacitor bank is switched to supply 500 kiloamperes to a blasting electrode positioned within a bore in a rock face causing dielectric breakdown of an electrolyte, preferably containing copper sulfate, to form a plasma. The electrolyte may be gelled with bentonite or gelatin to make it viscous enough so that it will not leak out of the confined area prior to blasting. The blasting apparatus has minimal inductance and resistance in order to reduce power loss and ensure rapid discharge of energy into the rock.
One of the drawbacks of the prior art systems is that the energy transfer from the capacitance to the explodable conductor or spark gap is relatively inefficiently. As a result of the inefficient transfer of energy, it was necessary to provide relatively large capacitor banks for driving either the explodable conductor or the spark gap to provide a given amount of explosive energy.
The spark gap systems also suffer from the draw-back that the zone at which the energy is to be dissipated, that is the gap between the electrodes, initially has a high impedance followed by insulating breakdown at the gap due to the applied voltage with a relatively lower impedance plasma being formed. As a result, the change in gap impedance from high to low impedance does not dissipate energy at the gap as efficiently as an exploding wire system might.